MPLS is a mechanism in high-performance telecommunications networks that directs data from one network node to the next based on short path labels rather than long network addresses. In an MPLS network, data packets are assigned labels. Packet-forwarding decisions are made solely on the contents of this label. This allows one to create end-to-end virtual circuits across any type of transport medium, using any protocol.
In MPLS network, routers that perform routing based only on the label are called label switch routers (LSRs). The entry and exit points of an MPLS network are called label edge routers (LERs). Labels are distributed between LERs and LSRs manually or by using the label signaling mechanisms, such as Label Distribution Protocol (LDP) and Resource Reservation Protocol-Traffic Engineering (RSVP-TE).
Generally, there are two traffic paths between two LERs, a working path (or working Label Switched Path (LSP)) and a protection path (or protection LSP), as illustrated in FIG. 1. In normal state, the Protection Switching Control (PSC) unit of both LERs controls the working path to transmit data therebetween. If there is a failure on the working path, then LER 110 and LER 120 will switch the traffic from the working path to the protection path and enter protecting failure state for the reason of signal failure (SF) on the working path. Once “SF clear” event on working path occurs, a wait-to-restore (WTR) timer is trigged. At this point, the LER 110 and LER 120 enter WTR state. If there is no SF on the working path, no admin intervention and no SF on the protection path, the WTR will expire (e.g. after 5 minutes) naturally, and the LERs will switch the traffic back to working path and enter normal state. And, if there is SF on the working path within the duration (WTR), LERs will terminate WTR and enter protecting failure state again. If there is SF on the protection path within the duration (WTR), LER will terminate WTR and enter unavailable state with traffic be switched back to working path immediately. Hence, a flap from normal (i.e. up state) to failure (i.e. down state) then back to normal on working path happens after WTR expire naturally, then a next cycle starts.
The disadvantage of the solution is that, if working path or both the working path and the protection path keeps flapping, the PSC will be busy on signaling and message handling, and in a scaled protection network, the frequent flapping may cause the LERs to be ramshackle. In addition, the working path and the protection path usually have different delay time and loss rate. If the traffic sways between the working path and the protection path frequently, some kind of services requiring stable service quality can not tolerate it.